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The Brfinsted-Lowry Acid-Base Concept George B. Kauffrnan California State University, Fresno, Fresno, CA 93740 For a number of reasons the Bransted-Lowry acid-base concept is a significant and integral part of the general chemistry course. Acids and hases are important classes of chemical substances, and the Bransted-Lowry concept forms a practical and useful compromise between the classical but too restrictive Arrhenius concept ( I ) and the somewhat too general (for students, a t least) Lewis (2) and Usanovich (3) concepts. Also, the approach outlined below, in which the Brdnsted-Lowry concept is considered by analogy with the usually earlier discussed electromotive force (EMF) series, enables the student to learn new material by proceeding from an already mastered concept. Furthermore, the Bransted-Lowry concept is a splendid example of simultaneous discovery, an occurrence in science that is much more prevalent than commonly believed.' Thus the BranstedLowry concept takes a prominent place among the better known cases of virtually simultaneous independent discoveries-what the father of the sociology of science, Robert K. Merton, has christened "multiples" (4); e.g., in mathematics the calculus (Newton and Leihniz), in biology the theory of evolution (Darwin and Wallace), in physics the law of conservation of energy (Mayer, Joule, Colding, Helmholtz, etc., (5)), and in chemistry the tetravalence and self-linking of carbon atoms (Kekul6 and Couper (6))and the periodic table (de Chancourtois, Newlands, Odling, Hinrichs, Lothar Meyer, and Mendeleev (7)). B$nsted
Johannes Nicolaus Bransted (8)was horn in Varde, Denmark, on February 22, 1879. He received his first degree in engineering (1899) from the Technical University of Denmark and his Maeister degree in chemistry (1902) and his doctorate (1908)f;om the University of copenhagen. He was a~oointedassistant in the university's chemical laboratory (i305) and professor of physical chemistry at the university (1908). He continued Julius Thomsen's idea of determining chemical affinity by measuring the maximum work of chemical reactions, hut, instead of using calorimetric measurements, he measured EMF values for galvanic cells, resulting in a series of monographs on chemical affinity (1906-1921). He also determined specific heats and affinity constants and published a series of studies on solubilities (1921-19231, the specific interaction of ions (1921-19271, the protonic concept of acids and hases (1923) (9),and catalysis (1924-1933). Bransted's growing international reputation attracted foreign co-workers, especially from 1921 to 1935. For example, with Gyorgy Hevesy he separated the isotopes of mercu-
' A detailed history of this concept will be dealt with in a subse-
quent ariicle. 28
Journal of Chemical Education
ry and chlorine, and with Victor K. La Mer he related activity coefficients to the ionic strength of solutions, a relationship derived theoretically by Peter Dehye and Erich Hiickel at about the same time. Bransted was unhappy with the classical formulation of the laws of thermodynamics, according to which heat is not directly comparable to other forms of energy. His formulations, especially those of work and heat, were not approved of by contemporary physicists, even by the time of his death. Bransted died in Copenhagen on December 17,1947. Lowry
Thomas Martin Lowry (10) was horn in Low Moor, Bradford, Yorkshire, England on October 26, 1874. He attended the Central Technical College, South Kensington and was awarded his DSc degree from the University of London in 1899. He was assistant to Henry E. Armstrong (1896-19131, Lecturer in Chemistry at the Westminster Training College (1904-1913), Head of the Chemical Department in Guy's Hospital Medical School (from 1913),and the first teacher of chemistry in a medical school to he made a professor at the University of London. In 1920 he was appointed to the newly established Chair of Physical Chemistry at Camhridge University. He died in Camhridge on November 2,1936. During his long service with Armstrong, Lowry became a master of organic technique, which gave him an advantage denied to many other physical chemists. He applied exact physical methods of measurement to the solution of chemical problems, and the vast amount of quantitative physical data collected by him and his co-workers provides useful
Johannes Nicolaus &$Isled (1879-1947) (left) and Thomas Martin Lowry (1874-1936) (right). Photos c o ~ r i e ~01ythe Oesper Collection. University of
Cincinnati.
working material for future generations of physical chemists. Lowry laid the foundation of his lifelong study of optical rotatory power early in his career by discovering the mutarotation (a term which he introduced) of nitro-d-camphor and the stereoisomerism of a number of halogen derivatives of camphor. In 1923 he proposed an extended definition of acids and hases, which was advanced independently by Brdnsted at the same time (11). In his work on the variation of optical rotatory power with wavelength, he realized that little nroeress could be made in the field until values were deterkinled over the widest possible range of wavelengths instead of for a sinele arbitrarily selected point on a disperhion curve. He intrf';duced the cdncept of i"dured dissymmetrv (19241.and in later sears he extended his work on rotatory dispersion from transparent to absorbing media, i.e., to a study of the Cotton effect. During World War I he investigated the production and use of amatol mixtures and the difficulties arising in shell filling caused by the polymorphism of ammonium nitrate. He also studied the chlorides of sulfur, pbosphorus(V) chloride, valency, and the binary systems N70q-H90and N10a-HgO. The foliowihg outline df the Br$nsted-Lowry concept is adapted from a handout distributed to the author's students during the last three decades (12).The students have found it very hebful in supplementinp the treatment in textbooks. ~cid-base-concepts-arealso diskssed in articles and monographs (13-22).
The Br$nsted-Lowry Concept Acids are substances whose aqueous solutions taste sour, change the color of certain indicators, e.g., hlue litmus to red, react with metals more active than hydrogen in the activity series to yield hydrogen gas, and neutralize hases to form salts. Bases are substances whose aqueous solutions are slippery to the touch and taste bitter, change the color of certain indicators, e.g., red litmus to hlue, react with amphoteric elements such as aluminum or zinc to form hydrogen gas, and neutralize acids to form salts.
Of the several concepts proposed to explain the properties and reactions of acids and hases, the one originated independently in 1923 by the Dane, Johannes Nicolaus Brdnsted, and the Englishman, Thomas Martin Lowry, is possibly the most useful. Accordine to this an~roach. which. unlike the .. older Arrhenius approach, is not limited to aqueous solutions but is a.~.d i c a h l to e all proton (Hf)-containine . . - svstems. " an acid is a proton donor, and a hase is aproton acceptor; the strength of an acid or base is correlated with its tendency to donate or accept protons, respectively. Acids may be positive ions (cations),neutral molecules, or anions, while bases may be negative ions (anions) as wellas neutral'molecules. A tahle of Brbnsted acids and their conjugate bases (the acids from which one proton has been removed) in order of decreasing acid strength is very useful. The acid-base table is similar to the already familiar activity (EMF) series and can he used similarly. Just as the EMF table consists of couoles. each consistine of a reduced and oxidized form. this acid-base table consist: also of couples, each consisting bf an acid and a hase (conjugate acid-base pairs). Just as the EMF series is conventionally given in terms of tendency toward electron loss (oxidation), so the acid-base tahle is conventionally given in terms of tendency toward proton loss (acidity). Just as Eovalues give the tendency toward electron loss, so K, values give thetendency toward proton loss. Just as the best reducing agents are found in the upper left-hand portionof the EMF table and the best oxidizing agents in the lower right-hand portion, so the strongest acids are found in the upper left-hand portion of the acid-base table and the strongest hases on the lower right-hand portion. The analoev between the EMF series and the BrdnstedLowry conceGt stems from the fact that both use half-reactions to store thermodynamic data, Eo or K, values, respectively. The procedure suggested here is based on the competition for a real or "imagined" intermediate, i.e., electrons The term "imagined Is important since these reactionsarereally two-step thermodynamic cycles, and no conclusions can be drawn about actual mechanisms. Indeed, many redox reactions in solution do not actually involve electron transfer but are the result of formal changes in oxidation state caused by atom transfer reactions.
Acld-Base Table Acid Perchloric Acid Sulfuric Acid Hydrochlnic Acid Nitric Acid Hydronium Ion 6 Oxalic Acid .- Hydrogensulfate Ion 5 Hydrafluaric Acid Hydrogenoxalate Ion 4 r: Acetic Acid .g Hexaaquaaluminum Ion
1
B c -
HCIO, t (Ht+) C104i H2S04e (Ht+) HS0.C HCI e (Ht+) CP HNOl t (Ht+) NO3H,O+ t (Ht+) H 2 0 H2C2O4a (Ht+) HC2O4-
HS0.i t (Ht+)s 0 r HF a (Ht+) FHC20r- e (H++)C20d2HC2H3O2 e (Ht+)C2Hs02AI(H~O)t ~ ~ (H++) + AI(OH)(H~O)~~+
H&O. t (H'+) HC03C HIS a (Ht+)HS-
5 Hypochlarour Acid 3 Ammonium Ion
HClO t (Ht+) CIONHIt FI (Ht+) NH3 HCN e (Ht+)CNHCOc e (Ht+)COs2H*0 e (H++) OHHS- e (Ht+) S2OH- t (Hi+) 02NH3 t (H++) NH2Ha t (Ht+) HCHI t (H++) CH1-
9: :2E
$ I
Hydrocyanic Acld Hydrogencarbonate Ian water Ion Hydrogen~ulflde Hydroxide ion Ammmia Hydrogen
I
Base
$ Carbonic Acid
;Hydrogen Sulfide 9
K.
Methane
Perchlorate Ion Hydrogensulfate Ion Chloride Ion Nitrate Ion Water Hydrogenoxalate Ion Sulfate Ion Fluoride Ian Oxalate Ion Acetate Ion Pentaaquahydroxw aluminum Ion Hydrogencarbanateion Hydrogensulfide ion Hypochlorite Ion Ammonia Cyanide Ion Carbonate Ion Hydroxide Ion SulfideIon Oxide Ion Amide Ion Hydride Ion Methide Ion
I f he aeivity of water is considered to be 55 M rather h e n 1 M in dilute aqueous solmion. K, would be =50 for H,o+. making ifastronger acidman HNO,(K. stions would result in a K. of =2 X to-" for water, making, H@ a weaker acid than nS- on a molar basis.
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= 1). Similar consider-
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29
or protons, respectively. Thus this approach can similarly he applied to the Lux-Flood acid-base concept (Lux, H. Z. Elehtrochem. 1939,45,303) or Ellingham diagrams, in which the intermediates are oxide ions or oxygen atoms, respectively. In our general chemistry course we discuss electrochemistry before acids and bases. For courses treating these topics in the opposite order the analogy still holds, but the procedure suggested should he reversed, i.e., the EMF series should he approached as analogous to the already discussed Br#nsted-Lowry concept. If we consider two given couples in the EMF table, the reducing agent of the couple higher in the table will reduce the oxidizing agent of the couple lower in the tahle, i.e., the "higher" reaction will go to the right, while the "lower" reaction coes to the left. Similarly, if we consider two given conjugate acid-base pairs in the acid-base table, the acid of the pair higher in the tahle will transfer a proton to the hase of the nairlower in the tahle. i.e.. the "hieher" reaction goes to the'right while the "lower" rkaction goes to the left In other words, in all acid-base reactions, stronger acids react to form weaker acids, while simultaneously, stronger hases react to form weaker bases. A reduction must always accompany an oxidation, since while the reducing agent gives up its electrons (is oxidized), the oxidizing agent accepts these electrons (is reduced). Similarly, in any acid-base reaction, the stronger acid gives up its protons, while the stronger hase simultaneously accepts them (a weaker acid and a weaker base thus being formed). Note that the table is arranged so thatconjugate acidbase pairs are opposite each other. Furthermore, coming down the table acids become weaker. but bases become stronger. Observe also that acid constants (KOvalues) hecome numericallv smaller coming down the tahle. Because the stronger the ~ r & t e d acid, the weaker the corresponding conjugate hase, and vice versa, the strongest acids appear in the upper left-hand portion of the table, and the strongest bases appear in the lower right-hand portion. An acid-base reaction consists of the transfer of a proton from an acid to a base. The acid therefore forms its conjugate hase, and the base forms its conjugate acid. A substance reacts as an acid only if a base is present to accept the proton of the acid. while a substance reacts as a base onlv if an acid is present to donate a proton to the base. An acid-base reaction is therefore a comoetition between two bases for a proton. If the stronger h of the two acids and the stronger of the two hases are reactants (appear on the left side of the equation), the reaction is said to proceed to a large extent:
+ Hz0 HCI hydrogen chloride water SA,
-
. SBz
H80t + C1hydronium ion chloride ion (1) WAz
strong acid (completely dissociated), while acetic acid is a weak acid. (Its aqueous solution consists largely of undissociated m~lecules.)~ Since any acid stronger than hydronium ion will donate its proton to water to form the hydronium ion, these acids appear equally strong in water (the so-called leveling effect), that is. the hvdronium ion is the stroneest acid that can exist in aqueous iolutiun. In order to dif&entiat~ hptween the streneths of diffrrrnt acids that are slroneer lhnn hvdron" um ion, a solvent that is a stronger acid than water must be used. In eqs (1) and (2) in which acids react with water, the water molecule behaves as a hase. When hases react with water, on the other hand, the water molecule behaves as an acid, as shown in the following two equations (A substance that can function as an acid in one reaction and as a base in a different reaction is said to he amphiprotic): 0,- + Hz0 oxide ion water
-
OH+OH- (or 20H-I hydroxide ion hydroxide ion
(3)
This reaction proceeds to a large extent, and therefore any metal oxide that dissolves in water gives a solution of the metal hydroxide. NH, + H,O ammonia water
-
NH,+ + OHammonium ion hydroxide ion
(4)
WB, WA, SA, SB2 The fact that this reaction proceeds only to a small extent agrees with the well-known fact that ammonia is a weak base. (Its aqueous solution consists largely of undissociated molecules.) Since any base stronger than hydroxide ion will accept a proton from water to form the hvdroxide ion, these bases Hppear equally strong in water (the leveling effect); i.e., the hydroxide ion is the strongest base that can exist in aqueous solution. In order to differentiate between the strengths of different hases that are stronger than hydroxide ion, a solvent that is a stronger base than water must be used. Hydrolysis (reaction of an ion of a salt with water to nroduce either HIOf or OH- ion) is simolv an acid-base ieaction and is therefore readily accounteci f;r according to the Brdnsted-Lowrv ao~roach.A salt will hvdrolvze to oro" " duce an acidic solucoi l'f it contains a cation that is a ~ & n sted acid; it will produce a basic solution if it contains an anion that is a Br#nsted base. If the cation is stronger as a Bransted acid than the anion is as a Bransted base, the resulting solution will be acidic. If this situation is reversed, the solution will be basic. The following three equations summarize the possible cases:
WB,
where the numerical subscripts are used to identify the corresponding conjugate acid-base pairs. If the weaker (W) of the two acids and the weaker of the two hases are reactants (appear on the left side of the equation), the reaction is said to proceed to only a small extent: H,Ot + C,H,O; HC,H,O, + H,O acetic acid water hydronium ion acetate ion (2,
-
-
Acidic Solution; Small Extent AI(H,o)$?+ + H,O hexaaquaaluminum ion water
WA,
A~OH(H,O)~ pentaaquahydroxoaluminumion
WB,
+
SB, (5)
H30+ hydronium ion
Equations (1) and (2) therefore agree with our common knowledge that in aqueous solution hydrochloric acid is a Notice that "strong" and "weak" are relative rather than absolute terms. In common oarlance the terms "strona" and "weak are used for acids that are stronger or weaker, respectively, than the nydronium Ion, 1.e.. the terms refer lo strength in aqueous solLlion, as snown in eqs (1) and (2). In so vents other than water (17)hydroch oric acid and acetic acid may behave as weak and strong acids, respectively. ~
~
~
30
~.~~~
~
~
Journal of Chemical Education
Basic Solution; Small Extent COa2- + H,O carbonate ion water WB,
WA,
-
+
HC0,OHhydrogencarbonate ion hydroxide ion (6) SA,
SB,
Neutral Solution; Small Extent NH,' + C2H30,ammonium ion acetate ion
-
NH, ammonia
+ HC,H30, acetic acid
(7)
SB, SA2 Anaqueoussolution of N H I C ~ H R is O neutral ~ even though it hvdrolvees because acetic arid is as strone an arid (K. = 1.8 X 10-5) as ammonia is a base (Kb = 1.8 X W*l
~~d~~
~. ~~
WB2
~~
Acknowledgment The author is indebted to Christian Klixbiill JBrgensen of the Universitb de GenPve for valuablediscussions and to the California State University, Fresno for a s a b b i l t i c a l leave of absence. Literature Clted ~
~
I. Arrhenius, S. Bihong t i l l Kongligo Suenrko Vetwskap8-Ahodemiem Hondlingor, trsnslsted into English 6 t h notes by Starkholm, 1884,8, Nos. 13 and 1.t: &ally Lodge, 0. B r i t . Assor. for the Advonerment of Scianrr Reports 1386,310, 357; 2. Physik. Chem. 1887, 1. 631: translated info English in Tha Foundofiom of the Th~aryofDi1utsSolufionr;AlembieClubReprintsNo. 19,Livmgstone: Edinburgh. 1961;pp4367.Forashort biographicalartielcaboutAnheniusseeKauffmen,G. B. J Chsm Ed""., in press. 3. Lewis, G. N. Valence and the Structure of Acorns on! Molecules: Chemical Catslog: New York, 1923. 3. Ussnovieh, M. Zhur. Obsheh. Khim. 1939.9.182.
4. Merfon,R. K. Pme.Arner. Phil. Soc. 1981,105(5), 470: reprintedin MaMn,R. K. The Sociology ofS&nea: Thear~Lieoland Empiricoi I n w ~ f i g o l i o n rUniversity : of Chicago: Chicago, 1973; pp 343-370. 5. Kuhn,T.S.In CnlicolP~oblernsinfheHisforyofSei~ne~;Clagett,M.,Ed.:Univeraity o l Wisconsin: Madison, 1959: pp 321-356. 6. Anschiltz. R. ''Uber sine neue chemisehe Thmrie "on Archibsld Scott Coupec"; Ostwald's Klmiker der erakten Wissenschsnen No. 183; Wilhelm Engdmsnn: hipiig, 1911; Kauffmsn,G.B. J.Chem.Edue. 1982,59,745. 7. van Spronwn, J. W. The Periodic System of Chemical Elements: A History of the First Hundred Years: Elsevier: New York, 1969: Csssebaum. H.; Kauffman, G. B. Isir 1971.62.314. 8. Chriatiansen, J. A. O~elaigtDonshe Videnrkobemea Selskobs 1948-1949.57; Ball. EL P. J. Chem Sot. 1910.403 Veihel, S. In Dictionor? of Seiantific Biography. Gilliapie, C. C.,Ed.: Scribne.: New York, 1970:Vol. 2, pp 49M99. 9, Br$nsted, J. N. Roe. Tmu. Chim. 1923,41,718; J . Phys. Chem. 1926,30,777; Chom.
...., ... .. 14. Bjerrum,N. Chsm.Reu. L935,16,287. 15. Bell, R. P.Quart. Rau. C h m . Soc. 1947,1,113. 16. VanderWert C. A. Acids, Bnsos, ond the Chemistry of the Cawlent Bond:Reinhold: New York. 1061. 17. Sisler, H. H.C h m ~ i s t r y i nNon-Aqueous Soluenta; Reinhold: New York. 1961. 18. Luder, W. F.: Zuffanti, S. Tho Electronic Theory of Acids end Bases, 2nd rev. cd, D o v u New Yoxk, 1961. 19. Bell. R. P. Acids ondBases Their @ o n l i t d u e Behouiour, 2nd ed.; Barnes 8 Noble: New York, 1971. 20. Bell, R. P. ThoPloton i n Chemistry. 2nd ed.:CorneU: Ithsca, NY, 1973. 21. Jensen, W. B. Tho Leuria Acid-Bnse Concepw A n Oueruiaw; Wlley: New York, 1930. 22. Finsfon, H. L.; Ryehtman, A. C. A Neu vielo of Current Acid-Boa= Theories; Wiley: New York. 1982.
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